The present invention relates in general to a microwave measuring method and apparatus, and in particular to a new and useful contactless and non-destructive testing of photosensitive materials and semiconductor layers, or components or circuits using such layers.
The present invention starts from the prior art such as disclosed for example, in the "Journal of Applied Physics", vol. 30, No. 7, July 1959, page 1054 ff (particularly pages 1057 to 1058). The starting point is that in a semiconductor material, an excess carrier density can be created by irradiation, and that the variations of the microwave field caused by the induced conductivity in this material which is placed in the field, can readily be measured. Consequently, an apparatus for this purpose comprise a waveguide system, a radiation source for creating the charge carrier excess in the photsensitive material, and more or less conventional measuring devices.
U.S. patent application Ser. No. 694,932 filed Jan. 25, 1985, assigned to the owner of the present application is incorporated here by reference and is also based on the possibility of determining the characteristic properties of the material through a nondestructive measuring method interrupting the manufacturing opertions only to an absolutely necessary extent. (At the time of invention, the subject of the present application and Ser. No. 694,932 were co-owned.)
In photoactive and photosensitive materials, the photosensitivity of course is one of the most important properties which can be tested also through a contactless excitation of charge carriers and through their influence on the surrounding microwave field, to infer therefrom the quality of the material.
Any change in the parameters (dielectricity constant, conductivity, etc.) in a waveguide system causes dispersion or absorption, or both, of microwaves. Determination of the parameters of a material from the variations of a microwave field under the influence of this material is known already for a long time (see, for example, "Dielectric Materials and Applications", by A. R. von Hippel, Wiley, N.Y., 1952 or "Measuring of Dielectric Properties of Materials" by A. Rost, Akademie Publishers, Berlin 1978). Already in 1953, these variations in the semiconductor layers had been used for determining the electrical transport properties of these materials (see "Phys. Rev. 89", by T. S. Benedict and W. Shockley, Jan. 16, 1953, p. 1152). The following measuring methods are usual:
(a) Measuring of variations caused by the material, of the proportion of standing waves; PA1 (b) Measuring of the absorption coefficient of the material; or PA1 (c) Measuring of the reflection coefficient of the material (see also "Rev. Sci. Instrum." Vol 44, No. 9, Sept. 1973, p. 1204 ff).
The methods under (b) and (c) permit a well defined evaluation of the parameters only under particular conditions, when the measurement is conducted at a single frequency. In general, the measurements must be numerically evaluated in a certain frequency region.
A measurement of the variations of a microwave field in a wave guide in which photosensitive material is received and exposed to irradiation with electrons or photons (excitation of the electrons beyond the band gap), makes it possible to determine the conductivity in excess produced in the material by the irradiation (see the above mentioned article in "J.Appl.Phys." Vol. 30, No. 7 July 1959). For various reasons, only methods under (b) and (c) are usable in this regard.
With an only slight change in the reflection and absorption coefficients caused by the excess conductivity, it may be shown that the relative variations under irradiation of the reflected and the absorbed microwave power are proportional to this excess conductivity (see "Radiat. Phys. Chem.", Vol. 10, July 1977 pp 353-365, or "Proc. IEEE" 51, 1963 sp. 681 ff). The constant of proportionality is a function of the material properties without irradiation (optical parameters and dimensions) and, in general, is evaluable if measurements can be made over a larger frequency region or with samples of different thickness. However, the evaluation is not needed if only relative variations of the excess conductivity are compared with each other, and the other equilibrium properties (properties without irradiation) of the material do not vary or vary only little. The measured excess conductivity then correlates with the property of the tested material in the photocells.